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Eprints ID: 6075
To link to this article: DOI:10.1016/J.CHERD.2010.06.001
URL: http://dx.doi.org/10.1016/J.CHERD.2010.06.001
To cite this version: Loubiere, Karine and Pruvost, Jérémy and Aloui, Fethi and
Legrand, Jack (2011) Investigations in an external-loop airlift photobioreactor
with annular light chambers and swirling flow. Chemical Engineering Research
and Design, vol. 89 (n°2). pp. 164-171. ISSN 0263-8762
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Investigations in an external-loop airlift photobioreactor
with annular light chambers and swirling flow
Karine Loubiere ∗ , Jérémy Pruvost, Fethi Aloui, Jack Legrand
Laboratoire GEnie des Procédés-Agroalimentaire-Environnement (GEPEA) UMR 6144, Université de Nantes - CNRS, CRTT, Boulevard de
l’université BP 406, 44602 Saint-Nazaire Cedex, France
a b s t r a c t
Photosynthetic microorganisms could serve as valuable compounds, but also for environmental applications. Their
production under controlled conditions implies to design specific reactors, named photobioreactors, in which light
supply is the main constraint. This paper was devoted to an original external-loop airlift photobioreactor (PBR) with
annular light chambers in which a swirling motion was induced. The aim was to characterize this novel geometrical configuration in terms of gas–liquid hydrodynamics, and to test its potentiality for algal cultures. This PBR
consisted of two identical columns connected by flanges defining tangential inlets, each column being made of
two transparent concentric tubes (6 L in liquid volume, 50 m−1 in specific illuminated area). Firstly, the global flow
characteristics (circulation and mixing times) were determined by a tracer method and modelled by an axial dispersed plug flow with complete recirculation (Péclet number). By means of a double optical probe, both local and
global time-averaged parameters of the gas phase were measured, namely void fraction, bubble velocity, frequency
and size. The gas–liquid mass transfer were also characterized, in tap water and in culture medium, by measuring
overall volumetric mass transfer coefficients. In a second time, cultures of the microalga Chlamydomonas reinhardtii
were run in batch mode. The variations of biomass concentration and pigment content with time from inoculation
were successfully obtained. All these findings highlighted: (i) some significant differences in terms of gas–liquid
hydrodynamics between the present PBR and the usual airlift systems, (ii) the interest of this configuration for algal
cultures, even if complementary studies and technological improvements are still required for definitively validating
its scale-up.
Keywords: Photobioreactor; External-loop airlift; Annular chamber; Swirling flow; Gas–liquid hydrodynamic and mass
transfer; Algal culture
1.
Introduction
Photosynthetic microorganisms, such as microalgae and
cyanobacteria, could directly serve as biomass (human food,
aquaculture) or indirectly as valuable compounds in the form
of pigments, vitamins or other food supplements, but also
for energy (biodiesel and hydrogen productions) and environmental (wastewater treatment) applications (Richmond,
2004; Spolaore et al., 2006; Chisti, 2007; Melis, 2007). Despite
many possible applications in various domains, industrial
developments are still limited, mainly for cost and productivity reasons. Indeed, the major barrier remains the difficulty
to develop intensive cultivation systems (named photobioreactor) with acceptable economic investments (in particular
when compared to heterotrophic cultures). All other nutrients
(like inorganic salts) being able to be supplied easily, the principal limiting factor is known to be light, as the radiation field
inside a photobioreactor (PBR) is highly heterogeneous due
to cell absorption and scattering. This is precisely what distinguishes PBR application from other classical bioprocesses
(fermentation in mixing tank for example). In an engineering
point of view, several ways are available to improve the productivity in PBRs: (i) to decrease the culture depth, i.e. to maximize
the ratio of illuminated surface to culture volume, (ii) to work
∗
Corresponding author at: Université de Toulouse, INPT, CNRS, Laboratoire de Génie Chimique, 4 allée Emile Monso, BP 84234, 31432
Toulouse Cedex 4, France. Tel.: +33 05 34 32 36 19.
E-mail address: [email protected] (K. Loubiere).
Nomenclature
Roman letters
a
gas–liquid interfacial area defined according to
liquid volume [L−1 ]
C
tracer concentration inside the reactor
[mol L−3 ]
C∞
final tracer concentration inside the reactor
[mol L−3 ]
CL
dissolved oxygen concentration [M L−3 ]
Dax
axial dispersion coefficient [L2 T−1 ]
Di
diameter of the inner column [L]
Do
diameter of the outer column [L]
DH
hydraulic diameter of the column (here Do − Di )
[L]
kL
mass transfer coefficient on liquid side [L T−1 ]
Lt
Mean circulation path length (Lt = 2.3 m) [L]
QG
volumetric gas flow rate [L3 T−1 ]
QL
circulating liquid flow rate defined by: QL =
VL (D2o − D2i )/4 [L3 T−1 ]
Ri
radius of the inner column [L]
Ro
radius of the outer column [L]
tC
circulation time [T]
tm95
mixing time defined using a homogeneity
degree of 95% [T]
tm99
mixing time defined using a homogeneity
degree of 99% [T]
VG
superficial gas velocity defined by: VG =
QG /[(D2o − D2i )/4] [L T−1 ]
Vl
liquid volume of the reactor [L−3 ]
VL
mean circulation velocity (or superficial liquid
velocity) [L T−1 ]
Greek letters
˛
void fraction
ˇ
gas
volumetric
fraction
defined
by:
ˇ = QG /(QG + QL )
S
maintenance term linked to respiration [T−1 ]
L
surface tension [M L−2 T−2 ]
ratio between time and mean circulation time
[Eq. (2)]
Dimensionless numbers
PeD
Péclet number based on the reactor diameter
(PeD = VL DH /Dax )
PeL
Péclet number based on the mean reactor
length (PeL = VL Lt /Dax )
z*
dimensionless geometrical distance between
tracer injection and detection points
Abbreviations
PBR(s)
photobioreactor(s)
PFD
photon flux density [mmolh L−2 T−1 ]
PPC
photo-protective carotenoids
std
mean
standard
deviation
defined
"
m
X
i=1
m
X
2
(Ciexp () − Cimod ()) /
i=1
2
(Ciexp ())
alternative path is to improve the light conversion by photosynthetic cells by cultivating specific strains or genetically
modified cells. These elements are the basis of any PBR design,
but their degree of integration is conditioned by the application and its own constraints (artificial or solar light, closed
or open systems . . .). As no ideal geometry exists, the stateof-art (Carvalho et al., 2006; Janssen et al., 2003) describes a
great diversity of geometrical configurations, among which
the most noteworthy are pneumatically agitated vertical column reactors, tubular reactors and flat panel reactors.
This paper focuses on the study of an external-loop airlift photobioreactor with annular light chambers in which a
swirling motion is induced. At present, such design is not
encountered in the available literature whereas it enables
to integrate inside one single device the advantages own to
pneumatic liquid circulation, annular geometry and swirling
flows:
- Airlift systems offer well-known advantages, making them
widely used for bioreactor applications (Chisti, 1989): simplicity of design and construction, lower power inputs, low
shear-stress field (respectful of the microorganism cellular
integrity), high gas–liquid mass transfer coefficients (supply
of CO2 and degassing of oxygen produced by photosynthesis);
- In annular geometries (Muller-Feuga et al., 2000), the light
sources are located within the culture, leading thus to minimise light input losses and improve specific illuminated
area (ratio of illuminated surface to culture volume);
- The mixing efficiency of swirling flows ensures a good
homogenisation of nutrients inside the culture, but also promotes the microalgae displacement and renewal along the
light gradient (Pruvost et al., 2002). In addition, the high
shear stresses generated at walls (Pruvost et al., 2004) are
interesting for limiting the biofilm formation at the optical
surfaces (Loubière et al., 2009).
This paper reports investigation carried out aiming at characterising this novel configuration of PBR and at testing its
potentiality for algal cultures. After describing the experimental set-up and methods, the global flow behaviour (circulation
velocity, mixing time and Péclet number) will be described and
compared with the results of Loubière et al. (2009) who studied the influence of swirling flow in a classical external-loop
airlift photobioreactor. The local and global parameters of the
gas phase (e.g. void fraction, bubble size and frequency), as
well as the gas–liquid mass transfer, will be next investigated
in tap water and culture medium. Lastly, batch cultures of the
microalga Chlamydomonas reinhardtii will be run, enabling thus
some concluding remarks for the scale-up of this PBR to be
drawn.
2.
Materials and methods
2.1.
Description of the experimental set-up
by:
#1/2
with high incident light flux with attention paid to photoinhibition phenomena, (iii) to optimize the hydrodynamic
conditions for promoting mixing while being respectful for the
fragility threshold of cells against hydrodynamic stresses. An
To test the potentiality of this novel geometrical configuration
of airlift reactors, a first prototype was built, with a total liquid
volume and a specific illuminated area equal to 6 L and 50 m−1
respectively. It was made of PMMA, as the walls of the reactor
should be transparent (visible domain) to allow an effective
light penetration (the culture was irradiated by means of two
fluorescent tubes, see Section 2.3). This device consisted of two
vertical columns of equal diameters, connected by two flanges
at their top and bottom (Fig. 1(a)). Each column was composed
Fig. 1 – (a) Schematic representation of the photobioreactor: [1], gas injection; [2], fluorescent tubes; [3], degassing volume;
[4], connection flange; [5], location for CO2 injection; [6], location for pH or O2 probe; [7], location for optical probe. (b)
Schematic representation of a connection flange (swirling motion generation): top view and front view.
of two concentric tubes (0.05 m in inner diameter Di , 0.08 m
in outer diameter Do ) defining an annular gap e of 0.03 m in
width (i.e. a depth of culture L of e/2). A degassing volume was
introduced at the top of the riser column, making the latter
higher than the downcomer column (1.2 m against 1 m).
When compared to usual airlift systems involving axial
flows, a swirling motion was here generated by means of
liquid tangential inlets inside both columns. For that, cylindrical connection flanges were designed with a diameter of
0.03 m, equal to the annular gap width e, and a length of 0.11 m
(Fig. 1(b)). To induce an efficient swirling motion (Loubière
et al., 2009), a velocity factor of 4.3 was chosen, the latter
parameter being defined as the ratio of the injection velocity at the flange outlet to the mean velocity at the flange inlet
(Legentilhomme and Legrand, 1995). Note that, for axial flows,
the velocity factor is equal to 1. A ring-shaped sintered porous
plate in stainless steel was used as air sparger. Air flow rates
QG were measured by a mass flow sensor (MEMS D6F-03A3000, Omron® ) and varied between 5 L h−1 and 120 L h−1 (i.e.
1.5 < VG < 11.2 mm s−1 ). They remained low when compared to
the literature on airlift reactors (Chisti, 1989). Indeed, higher
values were not wished in this specific configuration: they
would make the upper connection flange flooded and also
induce significant microalgal biomass losses due to bubble
spreading at the surface and entrainment by foaming.
2.2.
Methods for hydrodynamics and mass transfer
A conductimetric tracing method with two-measurement
points (Legentilhomme and Legrand, 1995) was applied to
obtain the Residence Time Distribution. The passive tracer
was an aqueous solution of NaCl at 50 g L−1 (pulse of 5 mL),
and was detected using two specially designed conductimetric cells. The latter were located in each connection flange and
made up of two semi-cylindrical nickel plates, insulated from
each other and having the same diameter than the connection
flanges. The time-variation in conductance was followed by a
conductimeter Tacussel® CD810 coupled with a data acquisition device. For each operating condition, experiments were
run from three to four times. The liquid phase was changed
after each three runs.
The characteristics of the gas phase were measured by
means of an optic probe technique. For that, a double-tip optic
probe (TTL) was used, integrating two simple optical probes
separated by a known distance. At the outlet of the opticelectronic modulus, both signals issued from the double probe
were acquired by a fast acquisition card (Keithley DAS-1802
ST). They gave access to raw phase indicator functions from
which, after thresholding and filtering, the time-averaged
void fraction ˛ were deduced. The other dynamic parameters required specific signal post-treatments (Aloui et al., 1999;
Aloui and Madani, 2008); the bubble axial velocity UB was
determined by temporal inter-correlations between both optical signals (e.g. mean transit duration of bubbles from a given
point of the first optrode to the second one). The bubble size dB
was calculated from the mean residence time in front of each
of both probes. Even if named dB , it is important to outline
that this parameter should be rather associated with a mean
bubble chord than a real bubble diameter, due to the bubble
shape and motion, but also to the way the bubble impacted
each probe (Chaumat et al., 2005). Whatever the conditions, the
measurement duration was fixed at 30 s with a sampling frequency of 20 kHz and a number of samples per probe of 600 000
points. Account for these parameters, the gas flow rate range
and the distance between probes, the bubble velocity and size
were measured with an error of 2–3%. For all experiments, the
double-tip probe was located in the upper part of the riser column (at 0.7 m above the gas sparger, Fig. 1(a)), the downcomer
column being free of bubbles.
The dynamic gassing-in method was used to measure
overall volumetric gas–liquid mass transfer coefficients kL a
(Roustan, 2003). After removing oxygen from the liquid phase
(nitrogen bubbling), air flow was established and the timevariation of dissolved oxygen concentration, CL , followed until
saturation. For that, an oxygen probe (Mettler Toledo O2 InPro
650) was placed inside the upper connection flange and connected to a transmitter (Mettler Toledo O2 InPro 4100). The
time constant of the oxygen probe, tP , was measured using
the method based on probe response to negative oxygen steps
(Vandu et al., 2004), and found equal to 14 s. This latter value
remained small when compared to mass transfer characteristic times, 1/kL a (higher than 63 s). As some fluctuations of
temperatures (19–23 ◦ C) were observed between experiments,
a temperature correction was applied (Bewtra et al., 1970):
kL a20 = kL aT 1.02420−T
(1)
Experiments were run in demineralised water issued from
an ion exchanger process, (conductivity: around 1 mS/cm;
pH: 7.03; L = 72.4 mN/m) and in algal culture medium
( L = 56.6 mN/m; see Section 2.3 for composition). The dissolved oxygen concentrations at saturation were then equal
to 9.3 mg L−1 and 8.4 mg L−1 respectively.
2.3.
Algal cultures
As well-known, the microalga Chlamydomonas reinhardtii (wild
type strain, coded 137 AH) was selected to evaluate the potentiality of the present PBR for algal cultures. The algal nutritive
medium was autotrophic, composed of (amounts in g L−1 ):
NH4 Cl (1.45), MgSO4 ·7H2 O (0.281), CaCl2 ·2H2 O (0.05), KH2 PO4
(0.609), NaHCO3 (1.68) and 1 mL of Hutner’s trace elements
solution. Before inoculation, an aqueous peroxyacetic acid
solution (5‰, 30 min) was used to sterilize the reactor. The
temperature was maintained close to 22 ◦ C (thermoregulated
room). pH was regulated at 7.5 by automatic injection of CO2 ;
for that, a pH sensor was placed in the upper connection
flange and CO2 was injected at the bottom of the downcomer
column (Fig. 1(a)). The light supply was ensured by two fluorescent tubes (Polylux XLR® , 30 W, Color Rendering Index
of 85, 0.026 m in diameter, 0.9 m in length) centrally placed
inside both inner tubes. The photon flux density (PFD) was
determined using a plane cosine quantum sensor (LICOR LI19 Lincoln® ; PAR: 400–700 nm; solid angle: 2). Average values
of PFD were deduced from measurements at various locations
on the surface of both inner tubes. Cultures were run in batch
mode (three tests) at a gas flow rate QG of 48 L h−1 . Biomass
concentrations, X, were daily measured by cellular counting (Malassez cell) and dry mass (weighting filtered samples
preliminary dried at 110 ◦ C during 24 h). The concentrations
of pigments (chlorophyll-a, chlorophyll-b and photoprotective carotenoids) were determined using a spectrophotometric
method (Strickland and Parsons, 1968); the pigment content
was then obtained as the ratio of pigment concentration to
biomass concentration (expressed in %).
3.
Results and discussion
3.1.
Hydrodynamics of the liquid phase
As in Benkhelifa et al. (2000), the global flow behaviour was
modelled by an axial dispersed plug flow with complete recirculation; it is based on the Voncken’s equation generalized
to reactors in which injection and detection are located at
different points (Takao et al., 1982):
Fig. 2 – Variation of mean circulation velocity VL versus
superficial gas velocity VG .
mization (Matlab® software) was considered valid if the mean
standard deviation, std, (between calculated and experimental
responses) was between 0.05 and 0.1.
The mean circulation velocity VL was defined by the ratio
of mean circulation path length Lt to circulation time tC . In
Fig. 2, it is reported as a function of superficial gas velocity
VG . Firstly, it can be observed that using tap water or culture
medium does modify VL . Thus, whatever the liquid phase, the
rise of VL with increasing VG can be modelled by an unique
law; the usual dependence in the form of a power law (Chisti,
1989) is here found again:
ˇ′
VL = ˛′ VG
(3)
where the exponent ˇ′ is equal to 0.55 ± 0.03 and the other
constant ˛′ to 1.06 ± 0.01 (std < 0.08). These values are fully in
agreement with the ones found by Loubière et al. (2009) for a
simple (i.e. not annular) external-loop airlift reactor equipped
with a membrane sparger and a velocity factor of 4.
Another way to comment the variation of VL with VG would
be to decompose this curve into two regimes characterized
by a slope break-up around a critical velocity of 5 mm s−1 (as
shown in Fig. 2). The first regime (VG < 5 mm s−1 ) would correspond thus to gas flow rates which are not sufficient to make
easy and complete the liquid circulation in the reactor, due
in particular to the strong pressure drops generated by the
connection flanges. In the second regime (VG > 5 mm s−1 ), the
latter resistance would be entirely overcome, ensuring thus a
fully free/established liquid circulation. This has been already
observed in Chisti (1989) and Olivo (2007).
Table 1 summarizes the other parameters characteristics
of the liquid phase. As for VL , they are identical for water and
culture medium (results not detailed here). For all gas flow
rates, the ratio of mixing time to circulation time remains
constant at 7.5 ± 1 for tm95 and at 10.7 ± 1.5 for tm99 . The presence of a swirling motion increases significantly the latter
values when compared to the ones commonly observed for
axial flows (between 2 and 4 as mentioned by (Chisti, 1989)).
The variations of tC , tm95 and tm99 with VG are successfully
described by various power laws for which:
(2)
- the exponent is always equal to −˛′ (i.e. to −1.06),
- the other coefficient is equal to Lt /˛′ for tC , to 7.5 Lt /˛′ for tm95
and to 10.7 Lt /˛′ for tm99 .
The mean circulation time, tC , was optimized, as well as the
Péclet number PeL and the geometrical factor z*, by fitting Eq.
(2) with the curve issued from tracer measurements. The opti-
In agreement with Loubière et al. (2009), no significant variation of the Péclet numbers with VG are observed: PeL = 89 ± 10
and PeD = 1.2 ± 0.1. These high values confirm that the swirling
C()
1
=
C∞
2
r
PeL X+∞
exp
j=−∞
PeL (j + z ∗ −)
−
4
2
Table 1 – Global parameters characteristics of the liquid phase [the associated experimental standard deviations are:
tC ± 2%, tm95 ± 8%, tm99 ± 6%, PeL ± 7% and PeD ± 6%].
QG (L h−1 )
17.4
29.7
47.9
59.1
71.4
90.9
123.9
VG (10−3 m s−1 )
tC (s)
tm95 (s)
tm99 (s)
PeL
PeD
87.7
55.6
41.9
37.3
35.4
29.6
25.0
583
354
344
272
241
251
184
852
538
506
404
346
370
238
85
79
102
87
86
97
75
1.1
1.0
1.3
1.1
1.1
1.3
1.0
1.6
2.7
4.3
5.4
6.5
8.2
11.2
flow inside the annular chambers tends towards a plug-flow
type behaviour. When compared to various reactor geometries, the present mixing properties behave rather as helical
or chaotic laminar flow systems (Castelain et al., 1997).
3.2.
Hydrodynamics of the gas phase
In Table 2 are collected the global (time- and cross-sectionalaveraged) parameters characteristics of the gas phase in
water. Firstly, the increase in mean void fractions <˛> with
VG is logically observed. It is mainly due to a rise of mean
bubble frequencies <fB >, as the mean bubble sizes and bubble velocities remain almost constant for all gas flow rates:
<dB > = 1.70 ± 0.2 mm and <UB > = 0.30 ± 0.05 m s−1 . Thus, for
this air–water system, the bubbles sizes are directly conditioned by the gas sparger.
The variation of <˛> with VG reveals also a discontinuity
around a superficial gas velocity of 5 mm s−1 . The radial distributions of void fraction seem to confirm this assumption: their
shape moves from flat to parabolic/domed around this critical
velocity (Fig. 3(a)). Such discontinuity is typically imputed to
a change in flow regimes, in particular from a homogeneous
regime to a heterogeneous one. Here, this explanation seems
inadequate as: (i) the gas superficial velocities applied remain
small (below 8 mm s−1 ), (ii) no significant changes in crosssectional-averaged bubble sizes with VG are observed, and (iii)
the slug regime is never visually observed. The bubbly or dispersed bubbly flow conditions are a priori conserved for all gas
flow rates. As a consequence, this discontinuity in <˛> should
be rather faced with to the two regimes previously outlined
(Fig. 2), namely to a change from a partial to a fully established
liquid circulation regimes.
As shown in Fig. 3(b), the radial distributions of bubble
sizes are almost uniform for all the superficial gas velocities.
On the contrary (Fig. 3(c)), the bubble frequencies are significantly higher in the centre of the annular gap; the bubbles
are then regrouped in the centre of the annular gap, where
the motion is more uniform and the shear stresses smaller
than near walls (Pruvost et al., 2000). For all gas flow rates,
the radial distributions of fB have the same shape: in partic-
ular, they are maximal for a dimensionless radial position of
0.45.
As shown in Table 2, the gas volumetric fractions ˇ, defined
by ˇ = QG /(QG + QL ) differ from <˛>. This suggests that the mean
void fractions measured 0.7 m above the gas sparger are not
representative of the whole annular sections along the riser
height, or in other words, that the axial profiles of the crosssectional averaged void fractions are not homogeneous. The
global correlations available in literature to estimate the gas
volumetric fractions write:
′
ˇ = a′ VGb
(4)
Chisti (1989) proposed a′ ≈ 1.45 and b′ ≈ 0.9 whereas the
present data lead to a′ ≈ 1.14 and b′ ≈ 0.48. Note that the latter exponent b′ agrees with the value of 0.5 usually found for
external-loop airlift systems (Bello et al., 1984; Choi and Lee,
1993). This kind of the correlations remains entirely empirical, and is thus hardly applicable to different geometries. To
improve this situation, Cockx et al. (1997) developed a simple relationship issued from a complete 1D model, based on
two-fluid mass and momentum balances (steady-state, nonaerated downcomer):
VG
ˇ=
√
G + jO ˇ
with jO =
r
2gHL
Ktot
and G =
VG
VL
−
(5)
ˇ
1−ˇ
The parameters involved in Eq. (5) offer the advantage to have
a clear physical meaning. Indeed, G is the slip velocity defined
according to Wallis (1969), and jO represents a velocity scale
which depends of: (i) the geometry of the reactor (liquid height
HL ), and (ii) the pressure drop coefficient, Ktot , related to the
areas connecting the downcomer and the riser columns. By
applying this model to the present set of data ˇ (VG ), a velocity scale jO of 0.18 m s−1 (namely a coefficient Ktot of 551) is
found. When compared to Cockx et al. (1997) (internal-loop airlift reactor), the value of jO appears small (0.18 against 1 m s−1 )
and the one of Ktot very high (551 against 8-12); this can be
explained by the strong pressure drops induced by the connection flanges implemented in this airlift reactor.
Table 2 – Global parameters characteristics of the gas phase (water) [they are calculated by integrating the radial
distribution of the time-averaged values along the annular gap thickness].
QG (L h−1 )
15.3
28.7
43.1
55.7
68.2
82.6
85.1
VG (10−3 m s−1 )
1.4
2.6
3.9
5.0
6.2
7.5
7.7
<˛> (%)
0.29
0.58
0.50
0.55
0.94
1.95
1.81
<UB > (m s−1 )
0.313
0.269
0.246
0.257
0.277
0.352
0.373
<fB > (s−1 )
0.83
1.48
1.30
1.43
2.34
5.30
4.99
<dB > (10−3 m)
1.68
1.69
1.43
1.51
1.72
1.97
1.96
ˇ (%)
4.9
6.7
8.1
9.2
10.1
11.0
11.2
Fig. 4 – Volumetric gas–liquid mass transfer coefficient
versus specific power input.
Fig. 3 – Radial distributions of time-averaged (a) void
fraction ˛, (b) bubble size dB and (c) bubble frequency fB .
3.3.
Gas–liquid mass transfer
The aeration performances are here evaluated in order to
get an idea of the quantity of oxygen physically transferrable
between gas and liquid phases in this PBR. This data will be
afterwards useful for scaling-up the reactor, in particular with
regard to the efficiency of degassing of the oxygen produced
photosynthetically by the microalga cultivated.
As demonstrated by Chisti (1989), the power delivered into
airlift or bubble column reactors comes mainly from the contribution of isothermal expansion of gas, leading to:
PG
≈ l gVG
Vl
(7)
The variation of the volumetric gas–liquid mass transfer coefficients, kL a, with PG /Vl is reported in Fig. 3 for water. It is
correlated according to the following relationship:
kL a = c ′
P d′
G
Vl
(8)
where the exponent d′ is equal to 0.60 ± 0.01 and the other
coefficient c′ to 7.3 ± 0.2 × 10−4 . Comparisons with conventional airlift reactors remain difficult because the present
system offers specific characteristics: small superficial gas
velocity (VG < 1 cm s−1 ), external-loop geometry with annular columns of equal sizes, swirling motion. An exception is
perhaps the airlift reactor described by Loubière et al. (2009),
which presents almost the same configuration except of being
not annular and using a membrane sparger. The comparison
with the results obtained by these authors reveals that the
exponent d′ of Eq. (8) has the same order of magnitude in
both cases, but the aeration performances are here improved
(Fig. 4). The velocity factor (FV) being almost the same, the
latter observation can be due to the difference in air spargers (and thus to the bubble sizes initially generated), and/or to
the effect of the annular confinement on the bubble break-up
phenomena.
Lastly, some measurements of volumetric gas–liquid mass
transfer coefficients are performed in the medium used for
cultivating the microalga Chlamydomonas reinhardtii. As already
presented in Section 2.3, this culture medium is composed of
various salts. Also reported in Fig. 3, the variation of kL a with
PG /Vl in culture medium can be successfully described by a
power-law-type relationship (Eq. (8)) having the same exponent d′ than for water (0.60 ± 0.01) and a coefficient c′ equal to
9.2 ± 0.2 × 10−4 . More interesting is that the values of kL a are
higher, from 1.1 to 1.3 times, in culture medium than in tap
water. It is mainly attributable to the decrease in bubble sizes
experimentally observed (even not quantified), involving thus
higher gas retention and interfacial area. This mechanism of
bubble size reduction should be linked to the presence of salts,
which could modify, as their concentrations are not negligible
(about 4 g L−1 ), the physico-chemical properties of water. The
measurement of the superficial tension of the culture medium
tends to confirm this assumption: a value of 55.6 mN/m at
20 ◦ C is found against 72.4 mN/m in water.
3.4.
Algal culture
The potentiality of this novel PBR for algal cultures is appreciated by running cultures of the microalga Chlamydomonas
reinhardtii in batch mode.
Fig. 5(a) presents the variation of biomass concentration X (expressed in terms of number of cells per mL and
of dry mass per liter) with time from inoculation (PFD of
141 mmolh m−2 s−1 ). The usual steps characteristics of the
batch culture dynamics are observed: the latency phase (t < 3
days), the growth phase (3 < t < 16 days), the stationary phase
Fig. 5 – Batch culture of Chlamydomonas reinhardtii
(PFD = 141 mmolh m−2 s−1 ): (a) Growth curve expressed in
terms of number of cells per mL and dry mass per L, (b)
Variation of the pigment content with time from
inoculation (experimental uncertainties of 13%).
(16 < t < 22 days) and the cell mortality phase (t > 22 days). An
experimental dry biomass concentration of 2.28 ± 0.23 g L−1 is
reached during the stationary phase. The dry mass productivity, rX , is usually deduced from the rates of change of the
biomass concentration during the growth phase: depending if
an average or a maximal value is considered, rX is here found
equal to 0.0074 or 0.0126 g L−1 h−1 respectively.
Fig. 5(b) presents the experimental variations of pigment
content with time from inoculation. Except for the mortality phase, they remain almost constant at 2.73 ± 0.19 for
chlorophyll-a, 1.21 ± 0.07 for chlorophyll-b and 0.77 ± 0.06 for
PPC.
During these experiments (three runs), the riser column
was kept totally free of biofilm thanks to the shearing action
of bubbles. In return, some foam is produced in the degassing
volume above the riser column (Fig. 1(a)). Commonly observed
in microalgal cultures (Richmond, 2004), this phenomenon
is due to the presence of specific molecules excreted by
microalgae, and is amplified in confined geometries where
small bubbles are generated. As the cultures were long (20–30
days), this foam should be broken to avoid biomass losses by
entrainment. For that, small amounts of an anti-foam product
(Antifoam B Sigma–Aldrich® , diluted at 1%, v/v) were added
by taking great care of disturbing neither the measurement
of biomass dry mass nor the characteristics of the gas–liquid
flow. Concerning the downcomer column, the optical wall
was maintained almost transparent even if a slight microalgal deposition can be observed at the walls of the inner tube
after 20 days. This confirms then the positive role of the
swirling motion. The three latter observations point out that,
in this kind of geometry, the gas flow rate to impose should
be carefully chosen, namely from a compromise ensuring a
swirling flow efficient both for limiting biofilm at walls and
for minimising foam formation at the surface. For long-term
continuous cultures (several months), some technical modifications should be integrated for mitigating these phenomena,
in particular a wide-mouthed shape of the degassing volume
and perhaps, depending of the type of the microalga cultivated, a self-cleaning system in the downcomer column (like
in (Hawrylenko, 2005)).
These preliminary tests outlined the interest of this novel
PBR and its potentiality for algal cultures. As in Loubière
et al. (2009), the scale-up of this PBR will be possible by
numbering-up the present geometry, that is to say by introducing several elementary modules which each outlet of
downcomer columns will be connected to the next module.
The internal illumination device implemented will enable
the same illuminated specific surface to be conserved whatever the numbers of modules, and thus, for a given incident
flux, the same volumetric productivity; this is an essential
advantage to avoid the usual constraints of scaling-up. As
multiplying the columns and connections flanges induces an
increase in investment costs, this type of photobioreactor does
not concern large-scale installations (i.e. solar systems), but
rather applications under artificial light, requiring few hundreds liters of culture volume. Further studies are still required
for definitively validating the practical feasibility of this scaleup by numbering-up (for example, the behaviour of the PBR at
higher photon flux density, especially with respect to biofilm
and foam) and also for evaluating the investment costs.
4.
Conclusions
This paper focused on the study of an original photobioreactor based on an external-loop airlift system with annular light
chambers (6 L in liquid volume, 50 m−1 in specific illuminated
area) in which a swirling motion took place. Specific investigations were carried out, aiming at characterizing the main
features of the gas–liquid hydrodynamics, but also at testing
its potentiality for microalgal cultures.
Tracer measurements gave access to the variation of the
mean circulation velocity as a function of the superficial gas
velocity, which was well-described by a power-law relationship. They also showed that the ratios of mixing time to
circulation times were higher to the ones usually observed
in airlift systems, and that high and uniform Péclet numbers
were obtained for all gas flow rates. Using an optic probe technique, the radial distributions of time-averaged void fraction,
bubble size and frequency were determined 0.7 m above the air
sparger. Coupled with the integrated values along the annular
gap thickness, they outlined that the increase in mean void
fractions <˛> with QG is controlled by the rise of the number of bubbles as the bubble size remains constant whatever
the gas flow rates. The variation of gas volumetric fractions ˇ
with VG was correlated using usual global correlations (powerlaw type) and the relationship proposed by Cockx et al. (1997)
in which parameters having a physical meaning are involved.
For design purposed (degassing of the oxygen photosynthetically produced), the aeration performances were quantified by
measurements of volumetric gas–liquid mass transfer coefficients. The values of kL a were successfully correlated with the
specific power input, and were higher in culture medium than
in water. Batch cultures of the microalga Chlamydomonas reinhardtii were run to test the potentiality of this novel PBR for
algal cultures. The usual four phases of the growth curve were
observed, a concentration of 2.28 g L−1 being reached during
the stationary phase (PFD = 141 mmol m−2 s−1 ). These preliminary tests of culture highlighted the presence of some specific
phenomena (like a foam formation in the degassing volume,
a slight microalgal deposit at walls of the downcomer).
This study confirmed the interests of this geometrical configuration for algal cultures, namely: the use of small gas
flow rates for liquid circulation (low energy input), the important gas–liquid mass transfer (O2 degassing), the production
of biomass with concentrations higher than 2 g L−1 at low
PFD (improved specific illuminated area), the minimisation of
the biofilm formation at walls (swirling motion). Some complementary studies and technological improvements are still
required for definitively validating the practical feasibility of
this scale-up by numbering-up (for example, the behaviour of
the PBR at higher photon flux density, especially with respect
to biofilm and foam) and also for evaluating the investment
costs.
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